HSC Physics Question Thread

[wyzard]

I have a question which i am getting close to the answe with, but not quite, and I have no idea why and I'm quite confused and frustrated.

The first part of the question requires you to find the Ep of a satellite of M=500kg, at 10,000km altitude.

i've subbed this into -G m₁m₂/r
hence:
6.67*10^-11* [500*(6.0*10²⁴)]/(6371000+10,000,000)  - Where 6371000 is the radius of the earth in metres and 10,000,000 is the altitude in metres.

But I get 1.222*10¹⁰J, rather than the answer; -3.119*10¹⁰J

Am I doing something wrong that i've missed or is the wrong answer written down?

You forgot the negative sign.

The only possible other place where there might be an error is that is the question referring to Earth or some other planet, since it is unspecified?

I've crunched the numbers and got the same answer as you did, so very likely the answer they'd given is wrong. Might want to verify with others in this forum though

[Rathin]

I have a question which i am getting close to the answe with, but not quite, and I have no idea why and I'm quite confused and frustrated.

The first part of the question requires you to find the Ep of a satellite of M=500kg, at 10,000km altitude.

i've subbed this into -G m₁m₂/r
hence:
6.67*10^-11* [500*(6.0*10²⁴)]/(6371000+10,000,000)  - Where 6371000 is the radius of the earth in metres and 10,000,000 is the altitude in metres.

But I get 1.222*10¹⁰J, rather than the answer; -3.119*10¹⁰J

Am I doing something wrong that i've missed or is the wrong answer written down?

Also got -1.22x10^10 J

[bsdfjnlkasn]

Hi,
Not sure if you need to know for HSC but
1. The centripetal (rotational force) = mv²/r, so as v increases, the centripetal force increases
When a planet rotates, the centripetal force = gravitational force
So if the velocity is increased, the centripetal force increases and therefore so must the gravitational force, and as Fg = GMm/r², r decreases.
Basically, it rotates faster when it's closer to the centre of the earth, and if it's closer then g is more.

Hey shadowxo,

Spoiler

Thanks for the detailed answer! We definitely learn about the centripetal force and how it is provided by the gravitational attraction force in space.

I'm a bit confused by how you connected centripetal acceleration with gravitational acceleration (g) in the last line. Is it just because the formula for g has the same variable r (or d because they represent the same distance) in it and because r decreases in the attraction force, it must also decrease for the gravitational acceleration? I'm just not really seeing the jump between the gravitational force and acceleration. Also, thank you for providing the 'extra' bit an explanation of work and Ep - they made perfect sense.

For anyone else who wants to help me out with my questions - I still have a few left 

1. I get really confused by angular velocity, my textbook first gives the definition for angular speed as

"Angular speed: How fast the angle of a line that joins the object with the centre is changing" and this is really confusing to me as I don't understand how this angle can change if an object travelling in a circle sustains tangential velocity. Doesn't this mean that the angle held at any instant by the circular-moving object and the centre is 90°?

2. What is an object's linear speed? If it increases, will it increase the object's angular speed also?

3. How do larger linear speed reduce the effects of gravity?

4. Is Earth's elliptical shape a result of it's rotational axis?

5. What provides Earth's centripetal force which allows it to orbit? Is it the gravitational fields of other, larger celestial bodies?

6. How does the linear orbit velocity acting on a satellite cause it to move away from the Earth?

Thank you 

[de]

For anyone else who wants to help me out with my questions - I still have a few left 

1. I get really confused by angular velocity, my textbook first gives the definition for angular speed as

"Angular speed: How fast the angle of a line that joins the object with the centre is changing" and this is really confusing to me as I don't understand how this angle can change if an object travelling in a circle sustains tangential velocity. Doesn't this mean that the angle held at any instant by the circular-moving object and the centre is 90°?

2. What is an object's linear speed? If it increases, will it increase the object's angular speed also?

3. How do larger linear speed reduce the effects of gravity?

4. Is Earth's elliptical shape a result of it's rotational axis?

5. What provides Earth's centripetal force which allows it to orbit? Is it the gravitational fields of other, larger celestial bodies?

6. How does the linear orbit velocity acting on a satellite cause it to move away from the Earth?

Thank you 

1. Angular velocity is easy to think about to start with as an average speed. So the average angular speed is defined as where T is the  time for one rotation. Instantaneous angular velocity is then where t is time (hopefully you're familiar with this notation) and it's just the angle of a really small arc, divided by the time taken to pass through it. So the "angle" they're talking about is the angle formed by the centre of the circle and two points on the circumference really close together.

2. If you mean by linear speed the tangential velocity, then yep (assuming the point stays the same distance from the centre of the circle).
If you mean the speed with which the centre is travelling. That is independent of the speed of a orbiting point.

3. Not quite sure what you mean here or what gravity you're talking about or what you mean by linear speed.

4. Again no idea. Surely the shape of the earth isn't a result of the fact you can draw a line through it? I might be being dense...

5. Yes other planets, the sun, etc, their fields act on the earth providing a radial acceleration and thus various orbits occur.

6. Linear velocity doesn't really "act" on a satellite. But, yes, if an external force acts on a satellite tangentially to its orbit then the velocity component in that direction increases and since v^2/r=a and the gravitational field is constant (a) r must also increase.

[Rathin]

Hey shadowxo,

Spoiler

Thanks for the detailed answer! We definitely learn about the centripetal force and how it is provided by the gravitational attraction force in space.

I'm a bit confused by how you connected centripetal acceleration with gravitational acceleration (g) in the last line. Is it just because the formula for g has the same variable r (or d because they represent the same distance) in it and because r decreases in the attraction force, it must also decrease for the gravitational acceleration? I'm just not really seeing the jump between the gravitational force and acceleration. Also, thank you for providing the 'extra' bit an explanation of work and Ep - they made perfect sense.

For anyone else who wants to help me out with my questions - I still have a few left 

1. I get really confused by angular velocity, my textbook first gives the definition for angular speed as

"Angular speed: How fast the angle of a line that joins the object with the centre is changing" and this is really confusing to me as I don't understand how this angle can change if an object travelling in a circle sustains tangential velocity. Doesn't this mean that the angle held at any instant by the circular-moving object and the centre is 90°?

2. What is an object's linear speed? If it increases, will it increase the object's angular speed also?

3. How do larger linear speed reduce the effects of gravity?

4. Is Earth's elliptical shape a result of it's rotational axis?

5. What provides Earth's centripetal force which allows it to orbit? Is it the gravitational fields of other, larger celestial bodies?

6. How does the linear orbit velocity acting on a satellite cause it to move away from the Earth?

Thank you 

Q4: When the Earth rotates around its axis it creates a apparent centrifugal effect which is greatest at the equator and less at the poles. This cohesive force causes the earth to 'bulge' at the equator due to the apparent value of g decrease and 'flatten' at the poles due to a increase in the value of g.

Q5: Gravitational Force is like a frictional force that provides the centripetal force. E.g When a car is going around a circle a centripetal force is provided by the friction on the road, similarly Centripetal force is provided by the Gravitational force thus we get the Fc=Fg (this is how the escape velocity formula is derived) 

EDIT: Fixed my silly mistake pointed out by Jamon

[Shadowxo]

Hey shadowxo,

Spoiler

Thanks for the detailed answer! We definitely learn about the centripetal force and how it is provided by the gravitational attraction force in space.

I'm a bit confused by how you connected centripetal acceleration with gravitational acceleration (g) in the last line. Is it just because the formula for g has the same variable r (or d because they represent the same distance) in it and because r decreases in the attraction force, it must also decrease for the gravitational acceleration? I'm just not really seeing the jump between the gravitational force and acceleration. Also, thank you for providing the 'extra' bit an explanation of work and Ep - they made perfect sense.

For anyone else who wants to help me out with my questions - I still have a few left 

1. I get really confused by angular velocity, my textbook first gives the definition for angular speed as

"Angular speed: How fast the angle of a line that joins the object with the centre is changing" and this is really confusing to me as I don't understand how this angle can change if an object travelling in a circle sustains tangential velocity. Doesn't this mean that the angle held at any instant by the circular-moving object and the centre is 90°?

2. What is an object's linear speed? If it increases, will it increase the object's angular speed also?

3. How do larger linear speed reduce the effects of gravity?

4. Is Earth's elliptical shape a result of it's rotational axis?

5. What provides Earth's centripetal force which allows it to orbit? Is it the gravitational fields of other, larger celestial bodies?

6. How does the linear orbit velocity acting on a satellite cause it to move away from the Earth?

Thank you 

For an object to be rotating around a planet, it has to be at a certain speed to maintain that rotation. That speed is faster as you get closer to the surface of the earth, so faster means closer to the earth. As you get closer to the earth, g increases. So a faster an object travels, the closer it is to the centre of the earth and therefore the greater gravitational force acting on it.
I think your other questions have been answered by these great posters 

[jamonwindeyer]

Q4: When the Earth rotates around its axis it creates a apparent centrifugal effect which is greatest at the equator and less at the poles. This cohesive force causes the earth to 'bulge' at the equator due to the apparent value of g increased and 'flatten' at the poles due to a decrease in the value of g.

Wait, isn't it \(g\) decreasing at the equator and compared to the poles? An apparent increase in gravitational acceleration would make things seem heavier, which would suggest things flattening compressing, so the poles should have the apparent increase right?

(Could have misinterpreted what you said, apologies in advance if I have)

[Rathin]

Wait, isn't it \(g\) decreasing at the equator and compared to the poles? An apparent increase in gravitational acceleration would make things seem heavier, which would suggest things flattening compressing, so the poles should have the apparent increase right?

(Could have misinterpreted what you said, apologies in advance if I have)

Omg I am so sorry, that's what I meant..accidentally wrote it the other way around!! The value of g is increased at the poles thus then being flattened and decreased at equator causing them to bulge.

[jamonwindeyer]

Omg I am so sorry, that's what I meant..accidentally wrote it the other way around!! The value of g is increased at the poles thus then being flattened and decreased at equator causing them to bulge.

Don't be sorry! All good - I know you know your shit so I literally said my post aloud before I posted it. I'm like, "I'm about to contradict Rathin, this is deep water."

[Aaron12038488]

Is Dot Point Preliminary Physics good?

[bsdfjnlkasn]

Hey De, Rathin, Shadowxo and Jamon (for the proof read)!

Thank you for these comprehensive answers, they've really cemented my understanding of the topics in question  . I'll be sure to ask more soon 

[Rathin]

Is Dot Point Preliminary Physics good?

Its okay..but some questions are really useless. For practice of questions I would suggest past papers>dot point books but they are not bad for passive practice.

[kiwiberry]

Is Dot Point Preliminary Physics good?

Hey! Some of my friends who used the dot point books said that they were helpful to review their learning after class. However, I know that others (including me) ended up being too lazy to do the questions regularly and never got around to using it. My phys teacher also said that the answers in the back are too brief and definitely not band 6 quality. I guess it's okay as a starting point and for revision, but personally I managed to survive on past papers haha I'm biased though so keep that in mind, you may find it useful!

[bsdfjnlkasn]

Q5: Gravitational Force is like a frictional force that provides the centripetal force. E.g When a car is going around a circle a centripetal force is provided by the friction on the road, similarly Centripetal force is provided by the Gravitational force thus we get the Fc=Fg (this is how the escape velocity formula is derived) 

Hey Rathin,

When you say gravitational force in Q5 do you mean gravitational attraction force because I have this in my notes: "The centripetal force which supports circular motion in space is provided by the force of gravitational attraction." Sorry if this is a silly clarification.

[Rathin]

Hey Rathin,

When you say gravitational force in Q5 do you mean gravitational attraction force because I have this in my notes: "The centripetal force which supports circular motion in space is provided by the force of gravitational attraction." Sorry if this is a silly clarification.

Yup exactly the same thing